1. Technical Field
Imaging electron energy filters are used in transmission electron microscopes in order to improve the contrast of object imaging or of diffraction diagrams by the selection of electrons of a given energy range. The recording of element distributions and energy loss spectra is also possible with such filter systems.
2. Background Art
Filter systems are known from German Patent document U.S. Pat. No. 5,449,914, U.S. Pat. No. 4,740,704 and U.S. Pat. No. 4,760,261 which use three or four homogeneous or inhomogeneous magnetic fields as dispersive elements. These energy filters are straight-vision, i.e., the optical axes of the incident and emergent electron paths are mutually coaxial. These direct-vision energy filters have the advantage of relatively simple adjustability, since the whole imaging system before and after the energy filter can be pre-adjusted with the energy filter switched off. This advantage is however achieved at the expense of a relatively large constructional height of the whole system of electron microscope and energy filter, since all the electron-optical components are arranged in series along a straight optical axis. Mechanical stability problems can arise from this, particularly with electron energies of 200 keV and higher, and with the relatively large filters, arranged asymmetrically with respect to the symmetry axis of the electron-optical column, which are required for these energies.
Moreover, energy filters are known, for example from U.S. Pat. No. 4,851,670, which have a single deflecting region as dispersive element, effecting a beam deflection through 90xc2x0. A single dispersive element however produces relatively large imaging aberrations, because of which a quite expensive imaging system has to follow the deflecting element. The 90xc2x0 deflection of the optical axis by the dispersive element, and the following horizontal course of the optical axis after the energy filter, admittedly reduce the constructional height. However, mechanical stability problems very easily arise with this system also, since the expensive optical imaging system after the energy filter leads to quite large moments under the influence of gravity.
Electron energy filters are furthermore known from German Patent document DE 198 38 600-A1 which likewise produce a 90xc2x0 total deflection of the optical axis between the filter input and filter output, but which nevertheless have a symmetrical structure with respect to the midplane by means of multiple beam deflection in opposite directions. It is known that the symmetrical structure of the energy filter enables some imaging aberrations to be avoided within the energy filter, so that improved imaging properties result overall. However, here also, the horizontal course of the optical axis after the filter output leads to mechanical stability problems.
The present invention has as its object an energy filter, particularly for electron microscopes, which on the one hand makes possible a small constructional height of the whole system of electron microscope with energy filter, and on the other hand leads to as few mechanical stability problems as possible. A further object of the invention is to provide an energy filter in which the imaging aberrations which arise electron-optically can be kept as small as possible.
The first-mentioned object is attained according to the invention by means of an energy filter with magnetic deflection regions wherein all the deflection regions in common produce a total beam deflection through an angle between 90xc2x0 and 210xc2x0, and the second-mentioned object is obtained by an energy filter with magnetic deflection regions which are arranged symmetrically with respect to midplane (M) and wherein the Helmholtz length of the energy filter is greater than double the average value of the deflection radii in the deflection regions. Advantageous embodiments of the invention will become apparent both from the combination of the two measures and also from the features of the dependent claims.
The electron energy filter according to the invention has several magnetic deflecting regions. All four deflecting regions in common produce a total deflection of between 90xc2x0 and 120xc2x0.
Because the total deflection of the optical axis between the filter input and filter output is more than 90xc2x0, an optical axis running obliquely upward after the filter output results when the optical axis runs vertically downward before the filter input. The moments arising under the effect of gravity on the electron-optical components arranged after the filter output are reduced by this obliquely upward course of the optical axis after the filter output. An optimum mechanical stability is of course attained when the optical axis runs vertically again after the filter output, and the filter thus produces a total beam deflection of 180xc2x0, deviations of the course of the optical axis by xc2x130xc2x0 from a vertical course having an only slight adverse effect on the mechanical stability. The limit of the maximum possible total deflection is given by the conditions that the detector following the energy filter must not be situated above the energy filter in the beam path of the electron microscope, and that the beam path emergent from the energy filter is also not to intersect the beam path entering the energy filter.
In order to keep the unavoidable electron-optical imaging aberrations of the energy filter small, besides maintaining mechanical stability, the energy filter is on the one hand to be constructed symmetrically with respect to a midplane, and at the same time the Helmholtz length is to correspond to at least twice, preferably at least three times or even five times, the average of the deflection radii in the deflection regions. The Helmholtz length is the distance between two planes, imaged to scale by the energy filter at a scale of 1:1, in or before the input-side region of the energy filter. One of these two input-side planes, the input diffraction plane, is then imaged dispersively at an imaging scale of 1:1 into the so-called dispersion plane, and the second of these two planes, the input image plane, is achromatically imaged at an imaging scale of 1:1 into the so-called output image plane.
A portion of the second order errors are known to disappear due to the symmetrical construction of the energy filter. By means of combination with a Helmholtz length which is long within the energy filter in comparison with the deflection radiixe2x80x94or with the average value of the deflection radii when the deflection radii are differentxe2x80x94a small ray pencil diameter results within the energy filter, so that it is furthermore attained that the unavoidable higher order imaging aberrations remain small.
A Helmholtz length which corresponds to at least five times the deflection radius or of the average value of the deflection radii is then particularly suitable for electron microscopes with a monocular head before the energy filterxe2x80x94seen in the direction of electron propagationxe2x80x94since the Helmholtz length then corresponds approximately to the usual constructional length of the monocular head, i.e., the distance of the last projective lens before the monocular head and the fluorescent screen or detector.
A beam deflection through an angle greater than 135xc2x0 preferably takes place in the first and last deflection regions. The energy filter has a very high dispersion because of the relatively long path lengths in the magnetic field associated with this.
It is furthermore advantageous if the first and last deflection regions respectively consist of two magnetic partial regions separated by a drift path, with the deflection angle in the first partial region after the filter input and in the last partial region before the filter output corresponding to the deflection angle of the two middle partial regions. At the same time, the drift path between the second partial region of the first deflection region and the second deflection region is to correspond to the drift path between the first and second partial regions of the first deflection region. The symmetry of the beam path in each of the two mutually symmetrical halves of the energy filter can thereby be further increased. The overall additional result is that there are beam paths close to the axis, and hence smaller errors of higher order.
In order to attain a maximum amount of symmetry, the two middle deflection regions are to be mutually separated by a drift path, the length of which corresponds to twice the distance between the input diffraction plane before the energy filter and the input edge of the first deflecting region. The energy filter then has a double symmetry, i.e., each of the two mutually symmetrical halves of the energy filter is itself furthermore symmetrical with respect to the midplane of the two halves, at least as concerns the focusing effect of the magnetic fields. Since the focusing produced by the magnetic fields is respectively quadratic with respect to the deflection produced by the magnetic field concerned, it is unimportant for the double symmetry of the energy filter that the two halves of the energy filter itself are themselves again symmetrical only up to a different sign of the deflection.
In order to attain an overall high dispersion and at the same time compact beam guiding, the deflection angle in each of the two partial regions of the first deflection regionxe2x80x94and of course correspondingly also in the two partial regions of the last deflection region which are symmetrical theretoxe2x80x94is to be greater than or equal to 90xc2x0. The deflection angle in the first partial region of the first deflection region is then preferably even between 110xc2x0 and 120xc2x0, ideally about 115xc2x0. The combination of a beam deflection of 115xc2x0 in the first partial region and a deflection of 90xc2x0 in the second partial region of the first deflection region then gives a maximum dispersion. A further increase of the deflection angle can only be realized with larger drift paths between the two partial regions of the first deflection region, since in other cases space problems arise between the second deflection region and the electron-optical components of the electron microscope.
In a particularly preferred embodiment, the energy filter is constituted as a whole as a telescopic or quasi-telescopic system. The energy filter is quasi-telescopic when the ratio of the objective focal length to the Helmholtz length about corresponds to the numerical aperture of the objective. This is attained when the Helmholtz length is greater than or equal to ten times the deflection radius or the average of the deflection radii. It has been found that the overall arising imaging aberrations can thereby be kept small, since the imaging aberrations of higher than second order remain small.
In order to completely correct the imaging aberrations of second order as a whole, it is advantageous to provide respective hexapoles in the drift paths between the individual deflection regions and partial regions; these are of course arranged mutually symmetrically in the two symmetrical halves of the energy filter. These hexapoles serve for the correction of the second order imaging aberrations.
In a portion of the deflection regions, the energy filter preferably has inhomogeneous magnetic fields.
The energy filter images an input-side plane, the input diffraction plane, dispersively into a freely accessible output-side plane, the dispersion plane or selection plane. A line focus, i.e., an image of the input diffraction plane focused in only one direction, thus arises in the symmetry plane of the energy filter.
Since the energy filter as a whole acts as a telescopic system, a real intermediate image of the object or of the sourcexe2x80x94according to whether an energy filtered object image or an object diffraction diagram is to be recordedxe2x80x94is to be produced in the focal plane, remote from the filter, of the last electron lens situated before the filter.
Particulars of the invention are described in detail hereinafter with the aid of the embodiment examples shown in the Figures.